The United States and Japan will step up their defence cooperation to deal with the threat from nuclear-armed North Korea as tensions in East Asia remain high, officials from the two allies said on Thursday.

Surge in ER visits for soccer injuries

Soccer injuries are sending soaring numbers of US kids to emergency rooms, a trend driven in part by young players with concussions seeking urgent medical care, a study has found.

The findings, based on 25 years of data, partly reflect soccer's growing popularity. But the researchers and sports medicine experts believe the trend also is a result of greater awareness about concussions and their potential risks. Coaches and parents are likely seeking emergency treatment for symptoms that in previous years might have been downplayed or overlooked.

Nearly three million players aged seven to 17 received ER treatment for soccer-related injuries in the 2000-2014 study.

The overall rate of injuries, which takes into account soccer's rising popularity, more than doubled to 220 per 10,000 players in 2013, from 106 per 10,000 players in 1990. Researchers did not have enough data to calculate 2014 rates.

More than 200,000 concussions or head injuries were treated in emergency rooms, or seven per cent of injuries. The rate jumped to almost 30 per 10,000 people in 2013 from just under 2 per 10,000 players in 1990.

That trend underscores a need for better safety education and injury prevention in youth soccer, said Dr Huiyun Xiang, the lead author and a researcher at the Centre for Injury Research and Policy at Nationwide Children's Hospital in Columbus, Ohio.

``Concussions can have significant consequences in terms of cognitive function and brain development,'' Xiang said.

His study is an analysis of data from a national injury surveillance system and was published on Monday in Pediatrics.

According to US Youth Soccer, there were more than three million registered soccer players younger than 19 in 2014, almost twice as many as in 1990.

Sprains and fractures were the most common injuries; most players were not hospitalised.

Falling and getting hit by another player or the ball were among leading causes of injury but there's no data in the study on whether many injuries were from heading the ball.

Concern about kids' injuries from heading led to the US Soccer Federation's recent restrictions including a ban for kids aged 10 and younger and limits on heading for those aged 11-13.

Many concussions in soccer occur when heads collide as two players jump up to head the ball, said Dr Cynthia LaBella, a sport medicine specialist at Chicago's Ann & Robert H. Lurie Children's Hospital. She said learning proper technique including tensing neck muscles can help kids avoid injury while heading the ball.

Labella noted that soccer has become so popular that it attracts kids with a wide range of athletic ability, and that many injuries she treats are in kids who lack adequate strength and conditioning for their soccer level.

``Everyone's so eager to join the soccer team. They're signing up because they want to be with their buddies,'' she said.

``The range of athleticism especially at the youth and pre-teen level is pretty broad.''